1. Field of the Invention
The invention relates to FM/CW or xe2x80x9cfrequency modulation/continuous wavexe2x80x9d type radio altimeters. These radio altimeters send out a continuous wave that is frequency modulated linearly between two boundary values. That is to say, it is a saw-toothed wave. When this wave is received by the radio altimeter after having been reflected off the ground, it has a delay of 2h/c with respect to the wave sent, where h is the height of the radio altimeter from the ground and c is the speed of light. Since the modulation is linear, this delay is proportional to the beat frequency, f, obtained by mixing the transmission signal and the reception signal. If the difference dF between the two boundary values and the time Td taken by the modulation to go from one boundary value to the other are known, then the height h can be calculated by the formula:
h=Td.f.c/2.dF
In reality, the mixing of the transmitted and received waves does not give the beat frequency. It gives an entire spectrum of frequencies from which it is possible, using different processing methods, to extract the frequency representing the height to be measured. Such a method is described, for example, in the French patent No. 2 750 766.
2. Description of the Prior Art
However, in these prior art radio altimeters, there is no way, apart from using testing stands, to monitor and check the quality of the transmission and reception antenna installations connected respectively to the transmission and reception receiver ports of the radio altimeter. Now, such an assessment would be very useful, especially in order to be informed of deterioration, as soon as it occurs, in an antenna installation or to take account more easily of parasitic echoes on obstacles linked to the radio altimeter.
The present invention is aimed at overcoming this lack of information on the state of the antenna installations, i.e. not only on the transmission and reception antennas and their connection cable but also on everything that surrounds them and affects, in varying degrees, the beat signal obtained by the mixing of the transmission and reception signals.
This result is obtained through the transmission, by means of the radio altimeter, of an incident signal to the installation to be checked and by studying the wave reflected by the installation concerned.
According to the invention, there is proposed, to this end, a method of checking designed for an FM/CW type radio altimeter having a transmission port and a reception port to which there are respectively connected a transmission antenna installation and a reception antenna installation, the radio altimeter comprising a linear oscillator to give a saw-toothed signal with a link to the transmission port, a mixing circuit with two links, respectively to the oscillator and the reception port, and a processing unit to process the output signals from the mixing circuit wherein, to check at least one of the two installations, the method consists of the transmission, from the radio altimeter to the installation to be checked, of an incident signal and the examination, in the processing circuit, of the signals reflected by this installation to be checked, this examination pertaining to the position and the amplitude of the lines of the spectrum of the signal given by the mixing circuit.
There is also proposed an FM/CW type radio altimeter comprising firstly, in series, a control unit, a linear oscillator and first coupling elements for the coupling of the oscillator to a transmission antenna installation and, secondly, a mixing circuit with a first input coupled to the linear oscillator and a second input, second coupling elements to couple a reception antenna installation to the second input of the mixing circuit and a processing unit to process the output signals of the mixing circuit wherein, to check one of the installations, it comprises first means to send an incident signal to the installation and second means to collect the signals received by the installation to be checked in response to the transmission of the incident signal and to send them to the second input of the mixing circuit.